It is known to provide a towing vehicle and a trailer with hydraulically actuated brakes, which brakes are operated over differing pressure ranges. The towing vehicle, which may be a farm tractor, can be provided with hydraulically operated brakes which are operated by a brake master cylinder over a relatively low pressure range. The trailer can be provided with hydraulic brakes operated over a substantially higher range. Due to the differences of pressures which may be required for actuating the brakes of the towing vehicle and the trailer, it is not practical to connect the brake line for the trailer directly into the brake line system for the vehicle. As most farm vehicles are provided with auxiliary hydraulics including a pump capable of delivering fluid under pressure at least equal to the requirements for the operation of trailer brakes, it is customary to provide a pilot operated trailer brake valve. This valve is interconnected with the pump and, when operated by pilot line pressure from the brake master cylinder, is capable of delivering fluid to the trailer brakes within a second pressure range substantially greater than the pressure range for actuating the vehicle brakes. An example of a prior art trailer brake valve is illustrated in U.S. Pat. No. 3,718,373 issued Feb. 27, 1973.
In the prior art designs the fluid delivered by the trailer brake valve has been used exclusively for the trailer and has not been available for the towing vehicle. In part this has been due to requirements that the braking of the towing vehicle should not be effected if the brake line to the trailer should break. However, if the output from the trailer brake valve could be utilized for the vehicle brakes the braking effort on the part of the vehicle operator could be substantially reduced.